The liquid layer lining the alveoli in the lung has been difficult to study because of its thinness, inaccessibility, and high water content. The electron microscope could magnify the alveolar lining sufficiently but its vacuum required that water be removed and the attendant shrinkage and distortion of this highly-hydrated layer made analysis problematic. With the advent of the low-temperature scanning electron microscope, lung can be analyzed at magnifications of 10 to 10,000X with water preserved as ice in the frozen hydrated state. The native air-liquid interface in the alveoli thus can be imaged directly. The proposed study will test several hypotheses: 1) That the surface lining layer is continuous over the entire alveolus and indeed over the entire alveolar duct portion of the respiratory zone will be shown by examining microdissections of airways in frozen hydrated rat lungs at high magnification. 2) That the contour of the surface is smooth will be tested by applying surface roughness criteria to stereo images of hydrated alveoli. 3) The thickness and volume of the aqueous subphase of the alveolar lining layer will be determined with photogrammetric methods from 3-dimensional images made with the low- temperature scanning electron microscope. 4) That all pores of Kohn are filled with alveolar lining material in normal lungs will be tested by counting the number of open pores in hydrated, frozen alveoli. Studies will be extended to human normal and emphysematous lung. Characterization of the alveolar lining layer is important in understanding the contribution of surface forces to the mechanics of respiration and for preventing and treating conditions that are disturbances of this layer such as the Respiratory Distress Syndrome, Pulmonary Edema and Pneumonia.